Since walls are preset, what if you are, say, men of the alpine, and are in an easily defensible position. Will the preset walls wall off choke points and expand your walls to the edge of the cliff, so you theoretically control the plateau and the few choke points leading up to it?

In the screenshot for the men of the alpines castle, the walls were built up to the edges of the cliffs, and the gate the only way onto the plateau with a bridge crossing a gap from the area just beyond the castle into the rest of the area. In the event of a siege, can you collapse the bridge, thereby restricting access to your plateau?

Well, you need to remember that siege weapons will be available, as in, rams can take down walls and stuff.

You can't effect the terrain (as in, make craters or something) so you can't really destroy land bridges, although I am not sure about normal bridges.